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KRN2011 - Johannes Gutenberg-Universität Mainz

KRN2011 - Johannes Gutenberg-Universität Mainz

KRN2011 - Johannes Gutenberg-Universität

On-Surface Covalent Linking of Organic Building Blocks on a Bulk Insulator Markus Kittelmann, † Philipp Rahe, † Markus Nimmrich, † Christopher M. Hauke, † André Gourdon, ‡ and Angelika Kühnle †,* † ‡ Institut für Physikalische Chemie, Johannes Gutenberg-Universität Mainz, Jakob-Welder-Weg 11, 55099 Mainz, Germany, and CNRS, CEMES, Nanoscience Group, 29 Rue J. Marvig, 31055 Toulouse, France Molecular self-assembly has been proven to constitute a powerful tool for creating tailor-made structures on surfaces. 1 Upon tuning the subtle balance between intermolecular and molecule surface interactions and by a rational design of the molecular building blocks, a multitude of structures have been constructed in clean and well-controlled ultrahigh-vacuum (UHV) conditions, ranging from perfect two-dimensional layers, unidirectional wire-like structures, 2 and well-defined clusters 3 to most complex architectures such as open networks 4 and host guest structures. 5 Molecular self-assembly requires reversibility of the involved supramolecular interactions to reach the (local) thermodynamic minimum. 6,7 This inherently required reversibility, however, constitutes a considerable drawback when aiming at chemically stable structures that bear the potential to be used ex situ in the harsh conditions of ambient environment. Moreover, when having molecular electronics applications in mind, the weak and reversible interactions pose a further challenge, as they hardly provide sufficient intermolecular electron transport capabilities. Only recently, the concept of on-surface synthesis 8 10 has been exploited as a promising strategy to overcome these limitations and obtain thermally and chemically stable structures by covalent bonding of suitable precursors directly on the substrate in UHV. 11 18 This strategy presents several other important advantages such as the possibility to prepare large molecules impossible to synthesize in solution due to their low solubility or new reactions not observed in solution by 2D confinement of molecular precursors. 9,18 So far, the few successful demonstrations of on-surface synthesis under UHV conditions have 11 17 been limited to metallic substrates, and only in one instance has a monolayer ABSTRACT On-surface synthesis in ultrahigh vacuum provides a promising strategy for creating thermally and chemically stable molecular structures at surfaces. The two-dimensional confinement of the educts, the possibility of working at higher (or lower) temperatures in the absence of solvent, and the templating effect of the surface bear the potential of preparing compounds that cannot be obtained in solution. Moreover, covalently linked conjugated molecules allow for efficient electron transport and are, thus, particularly interesting for future molecular electronics applications. When having these applications in mind, electrically insulating substrates are mandatory to provide sufficient decoupling of the molecular structure from the substrate surface. So far, however, onsurface synthesis has been achieved only on metallic substrates. Here we demonstrate the covalent linking of organic molecules on a bulk insulator, namely, calcite. We deliberately employ the strong electrostatic interaction between the carboxylate groups of halide-substituted benzoic acids and the surface calcium cations to prevent molecular desorption and to reach homolytic cleavage temperatures. This allows for the formation of aryl radicals and intermolecular coupling. By varying the number and position of the halide substitution, we rationally design the resulting structures, revealing straight lines, zigzag structures, and dimers, thus providing clear evidence for the covalent linking. Our results constitute an important step toward exploiting on-surface synthesis for molecular electronics and optics applications, which require electrically insulating rather than metallic supporting substrates. KEYWORDS: on-surface synthesis . surface chemistry . covalent linking . bulk insulator . noncontact atomic force microscopy . molecular electronics of NaCl(100) on Ag(100) been used. 19 However, for many applications such as molecular electronics, phase-supported organic catalysis, or molecular optics, it would be exceedingly attractive to transfer this technique to bulk insulators to prevent electronic coupling and leakage or nonradiative quenching. Covalent linking of organic molecules on a bulk insulator surface, however, poses additional challenges: The preassembly of the molecular building blocks is frequently hampered by the comparatively weak molecule surface interactions, leading to clustering at step edges and molecular bulk crystal formation. 20 Moreover, thermal activation of a coupling reaction is not feasible on many insulating surface, such as alkali halides, as most organic molecules would desorb at * Address correspondence to kuehnle@uni-mainz.de. Received for review August 28, 2011 and accepted September 7, 2011. Published online September 07, 2011 10.1021/nn2033192 C 2011 American Chemical Society KITTELMANN ET AL. VOL. 5 ’ NO. 10 ’ 8420–8425 ’ 2011 www.acsnano.org ARTICLE 8420

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